Process for producing sodium



"rates Unite This application relates to the electrolytic production of sodium and more particularly to a novel, improved, fused salt bath and process for such production at improved current cificiency.

Metallic sodium is usually obtained from the electrolysis of sodium chloride. This is the cheapest and most abundant salt of the metal but melts at too high a temperature for convenient use alone in electrolytic baths. As a consequence, it is generally employed in admixture with calcium chloride. The resultant mixtures melt at a temperature suiliciently low for commercial acceptance, and, upon electrolysis, yield sodium primarily. These mixtures generally contain a small percentage of barium chloride added as an impurity in the sodium chloride. As electrolysis proceeds, the concentration of this impurity in creases until an equilibrium is reached. A typical conventional bath consists principally of about 56-60% calcium chloride, 1-5 of barium chloride and a remainder of sodium chloride.

The aforesaid salt baths of prior practice have given good results, but, in general, operate at a current etficiency of 8085% (cf. Sodium, A.C.S. Monograph 133, p. 31, M. Sittig, Reinhold Publishing Corp, 1956). The cur-- rent efiiciency of these baths increases as the operating temperature is reduced. However, crust formation or any bath solidification interferes seriously with economical operation so that satisfactory results can be secured only when the baths are maintained at temperatures above the melting point. In view of the magnitude of the sodium in dustry coupled with the importance of sodium as a chemical intermediate and rising power costs, the development of a relatively low cost, readily operable bath of improved current efliciency is of outstanding industrial importance.

Alternate salt mixtures for use as electrolyte baths have been described in the prior art and, although some of these mixtures have shown improved current efiiciencies, this advantage has been offset by high material costs or production of sodium whose quality does not meet present high purity requirements and is not readily purified. Grahau, U.S. Patent 464,097 (Dec. 1, 1-891), disclosed a ternary mixture consisting of sodium chloride, another alkali metal halide and alkaline earth halides. This mixture was stated to have a current efficiency of 95% but yielded a sodium containing another alkali metal as an impurity. With the preferred other alkali metal salt, potassium chloride, the sodium was stated to contain 3% potassium. Seward et al., U.S. Patent 841,724 (Ian. 22, 1907) described a mixed salt bath containing sodium chloride, sodium fluoride and an alkali earth chloride. This bath gave sodium free of other alkali metals but its current efiiciency was not disclosed. However, neither of these baths appear to have found commercial acceptance. More recently, Cathcart et al., U.S. Patent 2,- 850,442 (Sept. 2, 1958) have devised a mixture consisting of sodium chloride, barium chloride and strontium chloride. This electrolyte yields sodium containing less than 0.1% impurities and shows current efilciencies of 85-89% which can be raised to 9095% by addition of 1-2% sodium fluoride. However, this mixture is expensive because of its strontium content in that the original investment and maintenance are of a relatively high order. Wood, U.S. Patent 2,876,181 (March 3, 1959), obtains high current efiiciency with a lithium chloride-sodium chloride electrolyte but the sodium is stated to contain ateut ice 4% lithium under exemplary operating conditions and a lithium electrolyte is expensive.

It has now been discovered that sodium having a relatively high degree of purity can be produced with current efiiciencies ranging from 88 to 97% by use of certain relatively low cost, ternary molten salt electrolytes containing calcium chloride and barium chloride.

An object of this invention is provision of a novel and useful method for producing sodium by electrolysis with direct current at a high current efficiency.

Another object is provision of a new method for the continuous electrolytic production of sodium at high current efficiency.

A further object is provision of novel and useful salt mixtures suitable for the electrolytic production of sodium at high current efiiciency.

A still further objective is provision of a novel fused chloride mixture containing calcium and barium chlorides for the production of sodium having a good degree of chemical purity at high current efficiencies, viz., in the range 88 to 97%.

The above-mentioned and still further objects of this invention are achieved by direct current electrolysis of certain fused salt mixtures comprising sodium chloride, calcium chloride and a balance consisting essentially of barium chloride. The composition of these mixtures in percent by weight is defined by a calcium chloride content in the range 15 to 30% and a sodium chloride content having a lower limit of 20%, an upper limit of 3 0% when the calcium chloride is less than 24% and an upper limit of 35% when the calcium chloride is 24% or higher. These compositions run higher in barium chloride than the eutectic (BaCl 31%; CaCl 49%; NaCl, 20%), which melts at approximately 454 C., and have melting points in the range 525 C. and above. For practical purposes, only those mixtures which have melting points of about 600 C. or below are normally employed for commercial sodium production. The fused salt mixtures of this invention are unique in that they make it possible to produce sodium at high current efiiciencies at cell operating temperatures in the range 545 C. to about 630 C. It has also been found that operating temperatures for electrolytic cells employing the fused salt mixtures of this invention should generally be about 15 C. higher than the melting or freezing point of the fused salt mixture employed. Operating temperatures should preferably lie in the range 545 to 630 C. Operating temperatures above 630 C. are not generally satisfactory for commer cial use.

An additional advantage inherent in the fused salt compositions of this invention lies in the fact that they have been found to yield a relatively pure sodium which is substantially free of barium and, depending on the specific compositions used, contain 0.5 to 3.0% calcium by weight.

The preferred compositions of this invention are defined by a calcium chloride content in the range 21 to 28% and a sodium chloride content having a lower limit of 24%, an upper limit of 29% when the calcium chloride'is-less than 25% and an upper limit of 32% when the calcium chloride is 25 or higher. The balance of these compositions consists essentially of barium chloride. The preferred compositions melt in the approximate range 550 to 580 C. These salt mixtures give the highest current etficiencies and yield sodium containing about 0.7 to 2% calcium. As a result of this low calcium content, cell operation is substantially free of the greater calcium co-deposition in the conventional Downs cell of U.S. Patent 1,501,756 (July 15, 1924). In this cell, for example, a solution of calcium in liquid sodium is formed and rises into a sodium receiver. As this solution cools, some of the calcium precipitates and drops back into the 3 bath. Since the precipitated calcium tends to block the pipe leading to the receiver, it is necessary to operate a stirring device to maintain sodium flow as described, for example, by Hulse et al., in US. Patent 2,068,681 (Ian.

'26, 1937). Precipitated calcium also tends to cause short circuits between the diaphragms and cathodes of the electrolytic cell, decreasing diaphragm life and cell efficiency. Since the sodium deposited from the preferred salt mixtures of this invention contains only about 1.5% calcium as compared with the 4 to 5% calcium of the conventional Downs cell, calcium precipitation in the cell is relatively negligible and the sodium can be removed continuously and at a higher temperature than in the conventionalv cell where cooling is necessary to precipitate excess calcium.

Due to its relative insolubility in sodium, calcium present as an impurity in sodium may be readily and economically removed by cooling the molten sodium to about 115 C. and then filtering. In this way it is readily possible to obtain sodium containing less than 400 ppm. calcium which meets ordinary commercial specifications for sodium. In the process of this invention, the crude sodium produced at high current efficiency can be removed from the cell without affirmative cooling and then purified by cooling followed by filtration. In such cases the sodium should be removed at a temperature above the saturation point for the concentration involved. For sodium containing 1.5% or less calcium, a temperature of 375 C. is satisfactory for this purpose. In the case of sodium containing 4% calcium a temperature of about 510 C. is required.

Sodium containing other alkali metals as impurities is not readily and economically purified since these metals are much more soluble than calcium and more expensive procedures are necessary for their removal.

The fused salt electrolyte mixtures of this invention contain not more than 30% sodium chloride when the calcium chloride content is less than 24% and not more than 35% sodium chloride when the calcium chloride content is over 24%. Experimental operation of cells employing fused salt mixtures containing 30 to 35% sodium chloride by weight, 12 to 24% calcium chloride and a remainder of barium chloride showed average current efiiciencies lower than that of the conventional Downs cell with a fused salt electrolyte mixture containing 56-60% calcium chloride, about 3% barium chloride and a remainder of sodium chloride.

The following examples are illustrative of the practice of this invention as compared with that of the conventional Downs-type cell containing a sodium chloride-calcium chloride electrolyte. All electrolyte compositions are reported as percent by weight.

Example 1 A substantially conventional Downs cell was charged with an electrolyte containing about 26% sodium chloride, 23% calcium chloride and a remainder of barium chloride. This bath had a melting point of about 560 C. and was operated with direct current for about 40 days-at an average temperature of about 605 C. This cell was equipped with a mesh diaphragm between anode and cathode and the spacing between these electrodes was 1.5 inches. Electrolysis was carried out at 30,000 amps. and a voltage potential varying from 7.1 to 7.3. The cell was found to deposit sodium at an average current efiiciency of 90%. A similar cell was also operated for 83 days with an average ourrent efficiency of 91.5%.

Example 2 A fused salt bath whose composition was maintained had an average freezing point of 590 C. Electrolysis was carried out at 31,500 amps. and at a voltage potential of approximately 6.5. This cell showed an average current efiiciency of 94%.

Example 3 A typical conventional sodium-calcium halide bath containing 5660% calcium chloride, 15% barium chloride and a remainder of sodium chloride was operated in a cell similar to that of Example 1 at. a bath temperature of 570-575 C. This electrolyte melted at about 550 C. This cell. produced sodium at a current efiiciency of 84.5% A similar cell operating at 590 C. deposited sodium at a current efiiciency of about 81.5% The results of this experiment not only demonstrate the low current efficiency obtained in the electrolytic production of sodium with the ordinary sodium chloride-calcium chloride electrolyte but also show that when the operating temperature or this electrolyte is raised to approach the temperatures of the electrolytic method illustrated in Examples l and 2, even lower current efficiencies result.

Example 4 A fused salt bath whose composition was maintained in the range of 29% to 31% sodium chloride, 25.5% to 27.5% calcium chloride and a remainder of barium chloride was electrolyzed for 106 days in a cell similar to that of Example 1. This electrolyte had an average freezing point of 575 C. and was operated at a temperature of about 595 C. Electrolysis was carried out at about 40,000 amperes and a voltage potential of approximately 7.1 volts. This cell showed an average current efiicie-ncy of 92%.

Example 5 A fused salt bath whose composition was maintained in the range of 32% to 34% sodium chloride, 26% to 28% calcium chloride and a remainder of barium chloride was electrolyzed for 8 days in a cell similar to that of Example 1. This electrolyte had an average freezing point of 590 C. and was operated at a temperature of about 610 C. Electrolysis was carried out at about 38,000 amperes and a voltage potential of approximately 7.0 volts. This cell showed an average current efliciency of 89%.

In the operation of the cells described in the above examples as in the case of all fused salt sodium cells, cell compositions must be maintained by feeding additional sodium chloride and smaller quantities of the other cell constituents.

This is a continuation-in-part of, my co-p'ending application Serial No. 832,465, filed August 10, 1959, now abandoned.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of producing sodium at high current efiiciency which comprises electrolyzing with direct current a fused salt mixture of sodium chloride, calcium chloride and a balance consisting essentially of barium chloride, said mixture having a percent by weight composition comprising a calcium chloride content in the range 15 to 30% and a sodium chloride content having a lower limit of an upper limit of when the calcium chloride is less than 24% and an upper limit of when the calcium chloride is 24 to 30%.

2. The method of claim 1 in which the operating temperature of the cell is in the range 545 to 630 C. and is about 15 C. above the freezing point of the electrolyte.

3. The method of producing sodium at high current efficiency which comprises electrolyzing with direct current a fused salt mixture of sodium chloride, calcium chloride and a balance consisting essentially of barium chloride, said mixture having a percent by weight composition comprising a calcium chloride content in the range 21 to 28% and a sodium chloride content having a lower limit of 24%, an upper limit of 29% when the calcium chloride is less than 25 and an upper limit of 32% when the calcium chloride is 25 to 28%.

4. The method of claim 3 in which the operating temperature of the cell is about 15 C. above the freezing point of the electrolyte.

5. The method of continuously producing sodium containing about 0.5 to 3% calcium at a current efficiency of at least 88% which comprises electrolyzing with direct current a fused salt mixture of sodium chloride, calcium chloride and a balance consisting essentially of barium chloride, said mixture having a percent by weight composition comprising a calcium chloride content in the range 15 to 30% and a sodium chloride content having a lower limit of 20%, an upper limit of 30% when the calcium chloride is less than 24% and an upper limit of 35% when the calcium chloride is 24 to 30%, at an operating temperature in the range 545 to 630 C. which is about 15 C. above the freezing point of the said fused salt mixture and continuously withdrawing sodium from said fused salt mixture as it is formed.

6. The method of claim 5 to which is added the additional step of removing the sodium from the fused salt mixture without aflirmative cooling means.

7. The method of claim 6 to which is added the steps of cooling the sodium to 115 C. after removal from the fused salt mixture and then filtering to reduce the calcium content of the said sodium to less than 400 ppm.

8. The method of continuously producing sodium containing about 0.7 to 2.0% calcium at a current efiiciency of at least 88% which comprises electrolyzing with direct current a fused salt mixture of sodium chloride, calcium chloride and a balance consisting essentially of barium chloride, said mixture having a percent by weight composition comprising a calcium chloride content in the range 21 to 28% and a sodium chloride content having a lower limit of 24%, an upper limit of 29% when the calcium chloride is less than 25% and an upper limit of 32% when the calcium chloride is 25 to 28%, in which the operating temperature is about 15 C. above the freezing point of the electrolyte and continuously withdrawing sodium from said fused salt mixture as it is formed.

9. The method of claim 8 to Which is added the additional step of removing the sodium from the fused salt mixture at a temperature of not less than about 375 C.

10. The method of claim 9 to which are added the steps of cooling the sodium removed from the fused salt mixture to about 115 C. and then filtering the said sodium to reduce its calcium content to less than 400 ppm.

11. A molten salt bath for the electrolytic production of sodium consisting essentially of a mixture of sodium chloride, calcium chloride and a balance consisting essentially of barium chloride, said mixture having a percent by weight composition comprising a calcium chloride content in the range 15 to and a sodium chloride content having a lower limit of 20%, an upper limit of 30% when the calcium chloride is less than 24% and an upper limit of when the calcium chloride is 24 to 30%.

12. A composition of matter consisting essentially of a mixture of sodium chloride, calcium chloride and a balance consisting essentially of barium chloride, said mixture having a percent by weight composition comprising a calcium chloride content in the range 15 to 30% and a sodium chloride content having a lower limit of 20%, an upper limit of 30% when the calcium chloride is less than 24% and an upper limit of 35 when the calcium chloride is 24 to 30%.

13. A composition of matter consisting essentially of a mixture of sodium chloride, calcium chloride and a balance consisting essentially of barium chloride, said mixture having a percent by weight composition comprising a calcium chloride content in the range 21 to 28% and a sodium chloride content having a lower limit of 24%, an upper limit of 29% when the calcium chloride is less than 25% and an upper limit of 32% when the calcium chloride is 25 to 28%.

14. A molten bath for the electrolytic production of sodium consisting essentially of a mixture of sodium chloride, calcium chloride and a balance of barium chloride, said mixture having a weight percent composition comprising a calcium chloride content in the range 21 to 28% and a sodium chloride content having a lower limit of 24%, an uper limit of 29% when the calcium chloride is less than 25 and an upper limit of 32% when the calcium chloride is 25 to 28% References Cited in the file of this patent UNITED STATES PATENTS 464,097 Grahau Dec. 1, 1891 2,850,442 Cathcart et a1. Sept. 2, 1958 2,898,276 Snow Aug. 4, 1959 

1. THE METHOD OF PRODUCING SODIUM AT HIGH CURRENT EFFCIENCY WHICH COMPRISES ELECTROLYZING WITH DIRECT CURRENT A FUSED SALT MIXTURE OF SODIUM CHLORIDE, CALCIUM CHLORIDE AND A BALANCE CONSISTING ESSENTIALLY OF BARIUM CHLORIDE, SAID MIXTURE HAVING A PERCENT BY WEIGHT COMPOSITION COMPRISING A CALCIUM CHLORIDE CONTENT IN THE RANGE 15 TO 30% AND A SODIUM CHLORIDE CONTENT HAVING A LOWER LIMIT OF 20%, AN UPPER LIMIT OF 30% WHEN THE CALCIUM CHLORIDE IS LESS THAN 24% AND AN UPPER LIMIT OF 35% WHEN THE CALCIUM CHLORIDE IS 24 TO 30%. 